Organic geochemical characterization of the Cenomanian–Turonian Eze-Aku (southern Benue Trough) and Campanian Nkporo Shales (Anambra Basin), southeastern Nigeria

Abstract

This study determined and compared, by means of total organic carbon (TOC) content analysis and Rock-Eval pyrolysis, the organic geochemistry and hydrocarbon source rock potentials of the Cenomanian–Turonian Eze-Aku (southern Benue Trough) and Campanian Nkporo Shales (Anambra Basin), southeastern Nigeria. A total of 30 samples, 15 each from both formations, were randomly collected and analyzed. The results indicate that the Eze-Aku Shale has average values of TOC (1.05 wt.%), hydrogen index, Hi, (253 mg HC/g TOC), S2 (3.2 mg HC/g), generative potential, Gp, (3.39 mg·HC/g), and pyrolysis temperature (Tmax) 436 °C that are comparatively higher than those of Nkporo Shale with TOC 0.81 wt.%, Hi 74 mg·HC/g TOC, S2 0.6 mg·HC/g, Gp 0.68 mg·HC/g, and Tmax 432 °C). Also, the average vitrinite reflectance of Eze-Aku Shale (0.67%) is higher than Nkporo Shale which is 0.59%. The organic matter is dominated by the type II and mixed type II/III kerogen for the Eze-Aku Shale and by a type III kerogen for the Nkporo Shale. The implication is that, although both formations have accumulations of marine-sourced organic matter types and would both rate as fair to good hydrocarbon source rocks (TOC > 0.50 wt.% which is considered the threshold value), the kerogen of Eze-Aku Shale is thermally more mature and is generally oil prone. Conversely, the kerogen of Nkporo Shale is within the immature to early mature stage, and is generally gas prone.